The present invention relates generally to a xerographic system, and, more particularly, to a system which incorporates a photoreceptor belt having a coded registration pattern on the surface thereof whose location is precisely detected for image registration purposes.
High speed copiers and digital printer machines typically employ a photoreceptor belt as the imaging media since the belt can provide significantly more area to form a plurality of latent images during a single pass or revolution when compared to a machine with a photoreceptor drum as the imaging surface. In a color copier, for example, a plurality of image exposures are formed, developed and transferred to an output paper sheet in either a single or multiple pass mode, to form a composite color output image. The position of the belt during operation must be known with a great deal of precision since the system timing control must ensure that the images are formed within preselected frames on the photoreceptor surface and each subsequent image is superimposed (registered) with previously formed images. The image exposures must be coordinated with the development, transfer and paper feeding functions. Registration errors occur because the belt is subject to changes in physical size and its rotational speed may vary because of mechanical wear and mechanical tolerances in the drive components. These errors are manifested in the output copies which may exhibit color separation, color bleeding and/or other defects which make the output copies unsuitable.
Various techniques have been developed in the prior art to compensate for these errors. One such method is to form holes or slits in the surface of the belt in non-image areas. The belt surface is illuminated with a light beam and light passing through the holes is sensed by a photodetector. Output signals from the photodetector are used to provide registration signals to register images on the belt in the process and cross process or scan direction and to correct for image skew. Another method is to form registration marks on the surface of the belt, either at manufacture or by exposing the belt in non-image areas to form the mark. The latent image of the mark can then be sensed or, alternatively, the latent image of the mark can be developed and sensed in conjunction with an illumination source or image detector.
The following patents are illustrative of these prior art registration techniques.
U.S. Pat. No. 5,175,570 discloses a color image forming apparatus which forms registration marks 1a, 1b, either as holes formed in the belt or as marks formed on the belt surface. The marks are then sensed and used to adjust the position of the latent image.
U.S. Pat. No. 5,208,633 discloses an imaging system which determines belt position by detecting illumination directed through fiducial openings in the belt.
U.S. Pat. 5,208,796 discloses a color system which detects the passage of either holes formed in the belt surface or marks formed on the belt surface to generate transverse error correction signals.
For these prior art systems, the instantaneous position of the hole or mark is sensed. Exact determination of the center or edge of the hole or mark within the desired accuracy of plus or minus 5-10 microns is difficult because of the "noise" factor. Noise is-attributable to stray light, variations in the registration mark and the distance between the photoreceptor belt and the detector. Also, for those systems using a laser Raster Output Scanner (ROS) as the imager, additional noise is created by asymmetry in beam shape, variation in the intensity of the laser source, and wobble of the rotating polygon. Another disadvantage of those techniques relying on hole detection is the belt degradation resulting from formation of the holes or slits; the belts tend to fray or tear during the hole-forming process or at the hole locations during use.
According to the principles of the present invention, a coded registration pattern is formed on the photoreceptor surface in a non-image area. A coded sensor incorporates a set of conductive strips formed in the same registration pattern. The sensor is set above the belt in a position to sense the coded registration pattern as it passes beneath. The sensor capacitively senses the coded latent image of the registration pattern and generates a signal which has a high correlation when the two patterns are exactly aligned. This signal (or plurality of signals when more than one sensor is used) generates signals for registration of the belt in the process, cross process and skew directions. These signals are much more accurate than the "noisy" signal generated by the prior art techniques and avoid the need to form openings in the belt with a resulting stress and fracture possible.
In a first embodiment, a coded line pattern is formed as a charge pattern latent image on one edge of a photoreceptor belt in a non-image area. The pattern can be formed as a latent image by the system imager, or laid down by an ion printhead or a separate image generator. In the latter two cases, the charge pattern may be applied to either the image side or the non-image side of the belt. Alternatively, the pattern may be coded on the surface permanently or periodically and charged by, for example, the charge corotrons of the electrophotographic system. A sensor is aligned in the process direction of the belt so that the coded pattern will pass there beneath. The coded charge pattern is capacitively coupled to the sensor, and when the pattern is exactly aligned with the sensor pattern, a coincidence signal is detected and used for both start of image registration as well as lateral registration. In a second embodiment, two coded patterns are formed at opposite sides of the belt. Two associated sensors generate correlation signals which can be compared to provide a correction signal for any skew in the belt or the image.
More particularly, the present invention relates to an electrographic printer, which incorporates a photoreceptor member which moves along a preselected path in a process direction,
imaging means for forming at least one image with a plurality of scan lines on the surface of said member, PA1 means for forming at least one coded registration pattern on the surface of said member, PA1 sensor means having a duplicate of said registration pattern formed on its sensing surface, said sensor fixed in position so as to be in periodic alignment with said coded pattern as said member moves in the process direction and to produce, by capacitive coupling, output electrical signals upon detection of said coded pattern passing there beneath and PA1 circuit means connected to the output of said sensor means for generating an output registration signal when said coded registration pattern is exactly aligned with said sensor means registration pattern.